1. Field of the Invention
This invention relates to systems and processes for liquefying natural gas. In another aspect, the invention concerns LNG processes and facilities employing an optimized heavies removal system.
2. Description of the Related Art
Cryogenic liquefaction is commonly used to convert natural gas into a more convenient form for transportation and/or storage. Because liquefying natural gas greatly reduces its specific volume, large quantities of natural gas can be economically transported and/or stored in liquefied form.
Transporting natural gas in its liquefied form can effectively link a natural gas source with a distant market when the source and market are not connected by a pipeline. This situation commonly arises when the source of natural gas and the market for the natural gas are separated by large bodies of water. In such cases, liquefied natural gas (LNG) can be transported from the source to the market using specially designed ocean-going LNG tankers.
Storing natural gas in its liquefied form can help balance out periodic fluctuations in natural gas supply and demand. In particular, LNG can be “stockpiled” for use when natural gas demand is low and/or supply is high. As a result, future demand peaks can be met with LNG from storage, which can be vaporized as demand requires.
Several methods exist for liquefying natural gas. Some methods produce a pressurized LNG (PLNG) product that is useful, but requires expensive pressure-containing vessels for storage and transportation. Other methods produce an LNG product having a pressure at or near atmospheric pressure. In general, these non-pressurized LNG production methods involve cooling a natural gas stream via indirect heat exchange with one or more refrigerants and then expanding the cooled natural gas stream to near atmospheric pressure. In addition, most LNG facilities employ one or more systems to remove contaminants (e.g., water, acid gases, and nitrogen, as well as ethane and heavier components) from the natural gas stream at different points during the liquefaction process.
In general, LNG facilities are designed and operated to provide LNG to a single market in a specific region of the world. Because specifications for the final characteristics of the natural gas product, such as, for example, higher heating value (HHV), Wobbe index, methane content, ethane content, C3+ content, and inerts content, vary widely throughout the world, LNG facilities are typically optimized to meet a certain set of specifications for a single market. In large part, achieving the stringent final product specifications involves effectively removing certain components from the natural gas feed stream. LNG facilities may employ one or more distillation columns to remove these components from the incoming natural gas stream. Oftentimes, the difference in relative volatility between the components being removed and the natural gas stream is small. In addition, at least one of the columns used to separate the undesirable components from the natural gas stream can generally be operated at or near the critical pressure of the components being separated. These limitations, coupled with the rigid product specifications, results in the distillation columns that are typically designed to operate within a relatively narrow range of conditions. When situations arise that force the column out of its design range (e.g., start-up of the facility or fluctuations in feed composition), the resulting unstable column operation may become unstable and may result in product loss and/or result in a LNG product that does not meet the desired product specifications.